1. Field of the Invention
The invention relates to a wheel bearing device which rotatably supports a wheel of an automobile on an car body.
2. Description of the Related Art
A wheel bearing device (hereinafter referred to as xe2x80x9cbearing devicexe2x80x9d) shown in FIG. 45 is for a driving wheel and has a constitution in which a hub ring 1, a bearing 2, and a constant velocity universal joint 3 are unitized.
The hub ring 1 is provided with an outboard inner raceway 4 on its outer peripheral surface as well as a wheel mounting flange 6 for mounting a wheel (not shown). Hub bolts 7 for fixing a wheel disc are studded on the wheel mounting flange 6 with equal intervals in a circumferential direction. A pilot portion 40 having a wheel guide portion 38 and a brake guide portion 39 is unitizedly formed with an outboard end portion of the hub ring 1. Also, a brake rotor (not shown) is mounted on the wheel mounting flange 6 together with the wheel disc.
The constant velocity universal joint 3 is provided at an end of a shaft 8 and comprises a joint outer ring 11 with a track groove 9 formed on its inside periphery, a joint inner ring 12 having a track groove 10 formed on its outside periphery opposing the track groove 9 of the joint outer ring 11, torque transmission balls 13 installed between the track groove 9 of the joint outer ring 11 and the track groove 10 of the joint outer ring 11, and a cage 14 supporting the torque transmission balls 13 disposed between the joint outer ring 11 and the joint inner ring 12. Between the joint outer ring 11 and the shaft 8 is a boot 15 for preventing entry of water and foreign matter from outside and leakage of grease contained inside.
The joint outer ring 11 has a mouse portion 16 storing the joint inner ring 12, the torque transmission balls 13 and the cage 14, and a stem portion 19 axially extending from the mouse portion 16, with a serrated portion 17 being formed thereon. Transmission of torque from the stem portion 19 to the hub ring 1 is enabled by inserting the stem portion 19 into a through-hole of the hub ring 1 so that they are fit each other through serrated portions 17 and 18 formed respectively on an outside peripheral surface of the stem portion 19 and an inside peripheral surface of the through-hole. An axial end of the joint outer ring 11 is plastically deformed to be roll-formed to an outboard end portion of the hub ring 1, and the joint outer ring 11 is fixed to the hub ring 1 by a roll-formed portion 20.
Double-row inner raceways are formed with an outboard inner-raceway 4 formed on an outer peripheral surface of the hub ring 1 and with an inboard inner-raceway 5 formed on an outer peripheral surface of a shoulder portion 21 of the joint outer ring 11. The joint outer ring 11 is inserted into the hub ring 1 axially from the inboard side and is roll-formed to the hub ring 1. Thereby a shoulder portion 21 of the joint outer ring 11 butts against an inboard end portion of the hub ring 1, thereby pre-load is applied to the bearing 2.
The bearing 2 is of a double-row angular ball bearing constitution, and is constituted such that rolling elements 25 and 26 are disposed between the inner raceways 4 and 5 formed respectively on outer peripheral surfaces of the hub ring 1 and joint outer ring 11 and outer raceways 23 and 24 formed respectively on an outer ring 22, and the rolling elements 25 and 26 of each row are supported by cages 27 and 28 at equal intervals in a circumferential direction. The outer ring 22 is provided with an car-body mounting flange 29 for mounting an car body (not shown) on it. The car-body mounting flange 29 is fixed with bolts on a knuckle extending from a suspension device (not shown) of the car body. At opening portion at both ends of the bearing 2, a pair of seals 30 and 31 sealing an annular space formed by the outer ring 22, the hub ring 1 and the joint outer ring 11 are fitted into inner peripheral portions at end portions of the outer ring 22 to prevent inside grease from leaking and water and foreign matter from entering from outside. The seals 30 and 31 are provided with seal lips that are in sliding contact onto outer peripheral portions of the hub ring 1 and the joint outer ring 11.
While the bearing device shown in FIG. 45 is a type having a non-separable constitution in which the hub ring 1, the bearing 2 and the constant velocity universal joint 3 are unitized together, FIGS. 46 and 47 show other examples of bearing devices of a type having a separable constitution in which a hub ring 1xe2x80x2 and the bearing 2 are unitized together, and the constant velocity universal joint 3 is fixed to the hub ring 1xe2x80x2 with bolts 32 or nuts 33. A separable type differs from a non-separable type as described below.
An inner ring 35, a separate element from the hub ring 1xe2x80x2, is fitted onto a small-diameter end portion 34 formed on an outside periphery of an inboard end portion of the hub ring 1xe2x80x2, and an inboard inner raceway 5 is formed on an outside periphery of the inner ring 35. The inner ring 35 is pressed into position with an appropriate interference to prevent creeping from occurring. Both an outboard inner raceway 4 formed on an outside periphery of the hub ring 1xe2x80x2 and the inboard inner raceway 5 formed on the outside periphery of the inner ring 35 form double-row inner raceways. The inner ring 35 is pressed onto the end portion of the small-diameter portion 34 of the hub ring 1xe2x80x2, the end portion of the small-diameter portion 34 of the hub ring 1xe2x80x2 is outwardly roll-formed by plastically deforming it, and the roll-formed portion 36 serves to prevent the inner ring 35 from loosening and coming off, and to apply pre-load to the bearing 2.
In this type of bearing devices, because of the constitution in which the roll-formed portion 36 serves for the prevention of coming off and application of pre-load, the joint outer ring 11 is fixed to the hub ring 1 in the following way: a stem portion 19 of the joint outer ring 11 is inserted into the through-hole of the hub ring 1xe2x80x2, then the joint outer ring 11 is fixed to the hub ring 1xe2x80x2 with tightening torque necessary and sufficient to prevent the joint outer ring 11 from loosening from the hub ring 1xe2x80x2 either by engaging a bolt 32 into a threaded hole 37 formed on a stem portion 19 of the joint outer ring 11 (see FIG. 46) or by engaging a nut 33 onto the stem portion 19 of the outer joint ring 11 (see FIG. 47).
The bearing device in FIG. 45 is constituted such that the stem portion 19 of the joint outer ring 11 is roll-formed by plastically deforming it, and the joint outer ring 11 is fixed to the hub ring 1 by a roll-formed portion 20. Therefore, considering the convenience in assembly of this bearing device, a serration fit between the hub ring 1 and the stem portion 19 of the joint outer ring 11 is preferably loose.
Also, in the bearing devices in FIG. 46 and FIG. 47, the roll-formed portion 36 of the hub ring 1xe2x80x2 serves to prevent the inner ring 35 from loosening and coming off, and to apply pre-load to the bearing 2, and the bolt 32 or the nut 33 fixes the constant velocity universal joint 3 to the hub ring 1xe2x80x2. Because the swaged portion 36 of the hub ring 1xe2x80x2 serves to prevent the inner ring 35 from loosening and apply pre-load to the bearing 2 as described above, applying pre-load by fastening torque of the bolt 32 or the nut 33 becomes unnecessary, and the bolt 32 or the nut 33 fixes the joint outer ring 11 to the hub ring 1xe2x80x2 with fastening torque that is necessary and sufficient to prevent the joint outer ring 11 from loosening.
However, when the serration fit between the hub ring 1 or 1xe2x80x2 and the stem portion 19 is loose for convenience in assembly of the bearing device, play may occur between the serrated portions 17 and 18 of the hub ring 1 or 1xe2x80x2 and the stem portion 19 respectively, resulting in possible deterioration in drive feeling and generation of an unusual sound in a driving system. Also, a constitution of this type is weak to moment load applied to the hub ring 1, an attempt for securing sufficient strength for the stem portion 19 and the roll-formed portion 20 hinders the miniaturization of the entire device.
Also, because the bearing devices in FIGS. 45 to 47 are constituted such that the stem portion 19 of the joint outer ring 11 is fitted into an inside periphery of the hub ring 1 or 1xe2x80x2, the dimensions in a radial direction of the serrated portions 17 and 18 for torque transmission cannot be made larger than the inside diameter of the hub ring 1 or 1xe2x80x2. When the diameters of serrated portions cannot be made larger as described above, the serrated portions inevitably have to be made longer in an axial direction to secure a predetermined transmitted torque, which results in inconvenience in that the dimension in the axial direction of the bearing device increases.
In the bearing device in FIG. 45, specifically, moment load acting on the hub ring 1 is received mainly only by the outboard bearing 2 of the double row bearing 2. This is because the hub ring 1 and the joint outer ring 11 tend to be bent at their butted portion when moment load is applied, so that an inboard bearing is not capable enough to support moment load. Therefore, excessive force may cause looseness at the roll-formed portion 20, so that lack in coupling strength between the hub ring 1 and the outer joint ring 11, as well as lack in strength of the stem portion 19 of the joint outer ring 11 is feared.
Also, in the bearing devices in FIGS. 45 to 47, because the inner raceway 4 is formed on the hub ring 1 or 1xe2x80x2, and the hub ring 1 or 1xe2x80x2 has a function equivalent to a raceway of a general roller bearing, the hub ring 1 or 1xe2x80x2 must be made of steel for bearing in the same way as a raceway of such a general roller bearing. However, because steel for bearing is extremely purified steel, it is expensive. Also, steel for bearing contains a higher amount of carbon for hardenability and for hardness on a raceway surface, so that it has low ductility, which leads in poor workability in forging.
In a wheel bearing device, generally, an inboard bearing is subjected to severer load conditions than an outboard bearing. Conventionally, specifications of internal parts of both inboard and outboard bearings have been made equal without considering such a point described above. Briefly, the pitch circle diameters and other dimensions of outboard and inboard bearings have been made equivalent. This means, however, that an outboard outer raceway 23 also inevitably has a large radius dimension that satisfies the rated load of an inboard bearing, which is against demand for miniaturization and weight-reduction of a bearing device. Furthermore, when designing a bearing device, it must be considered that a hub bolts 7 do not contact with an outer ring 22 in case of repair, for example, in the case when the hub bolt 7 is drawn from wheel mounting flange 6; however, a larger radial dimension of the outboard outer raceway 23 makes such consideration difficult to realize, restricting the freedom in design. Consequently, the design itself of a bearing device cannot be realized in an extreme case.
In a bearing device in FIG. 45, a axial end of the joint outer ring 11 is plastically deformed to roll-form it to an outboard end portion of the hub ring 1, and the joint outer ring 11 is fixed to the hub ring 1 by the roll-formed portion 20. Therefore, during the roll-forming operation, run-out occurs on the wheel mounting flange 6 of the hub ring 1. Run-out of the flange will cause face run-out (or run-out in an axial direction) on a brake rotor to be mounted on the wheel mounting flange 6 of the hub ring 1, causing vibration in braking while the automobile is traveling at a high speed or resulting in a problem such as uneven wear of the brake rotor or brake juddering.
Generally, in view of the reason of easiness in forging, workability in cold forging, and machinability or because of economy, raw un-heat-treated medium carbon steel (S53C and others) for machine structural purpose is used for the hub ring 1. Miniaturization and weight-lightening of a bearing device greatly contribute for increased travel stability of an automobile, so that the wheel mounting flange 6 of the hub ring 1 is increasingly constituted with ribs and thinned. However, such movement is bringing the mechanical strength of the hub ring itself to a fatigue limit of the material, or medium-carbon steel for machine structural purpose, and therefore, further weight-reduction is becoming difficult. Specifically, thinning of the wheel mounting flange 6 of the hub ring 1 for weight-reduction purpose can cause concentration of rotational bending stress at an outboard base portion of the wheel mounting flange 6, or at a fillet located at a region extending from a brake rotor mounting surface to a cylindrical pilot portion 41, and the fillet can be a starting point of breakage.
An inboard base portion of the wheel mounting flange 6 is a sealing surface with which the seal-lip of a seal 30 is in sliding contact, and the sealing surface has a larger curvature with a quenching-tempering treatment being applied to give abrasion resistance on the surface. Therefore, the inboard root portion of the wheel mounting flange 6 has higher mechanical strength than an outboard base portion that is not heat-treated, so that the inboard base portion is less likely to be a starting point of breakage caused by rotational bending stress.
Although thickening of the wheel mounting flange 6 can be a solution for avoiding such breakage described above, it is against the movement of weight-reduction. Further, generation of stress can be eased with enlarged dimensions of the base portion, or the curvature of the base portion, of the wheel mounting flange 6; however, the application of this method is limited by a possible mechanical interference between the base portion and a brake rotor to be mounted on the wheel mounting flange 6.
Also, material can be strengthened by increasing its carbon content, adding a strengthening element such as Si (silicon) or V (vanadium) or by applying a heat treatment such as normalizing; however, workability of the material is affected by increased material hardness. Consequently, conventional processing methods or existing facilities become difficult to be applied, and further, adding a large amount of strengthening elements leads to increased material costs.
It is an object of the present invention to prevent occurrence of play between serrated portions of a hub ring and a joint outer ring.
To attain the above object, a bearing device according to the present invention comprises a hub ring, a double-row bearing and a constant velocity universal joint, at least the hub ring and the double-row bearing are unitized, and the hub ring and a joint outer ring of the constant velocity universal joint are fitted to each other through torque transmission means and joined together by means for fastening in an axial direction. In this constitution, a clearance in the fit portion of the torque transmission means is prevented from occurring by a play prevention member.
In this bearing device, fit condition of the torque transmission means provided between the hub ring and the joint outer ring, which was in a loose relationship for convenience in assembling a bearing device, can be changed to a tight relationship because clearance in the fit portion of the torque transmission means provided between the hub ring and the joint outer ring is prevented from occurring by the play prevention element. Thereby, occurrence of play between the serrated portions of the hub ring and the joint outer ring, deterioration of drive feeling and generation of unusual sound from a driving system can be prevented so that a highly reliable bearing device having high quality can be provided.
This invention is also applicable to a bearing device having a constitution in which a hub ring, a constant velocity universal joint and a double-row bearing are unitized, one of double-row inner raceways of the bearing is formed on a joint outer ring of the constant velocity joint, and an end portion of the joint outer ring is fitted onto an outside periphery of the hub ring through torque transmission means.
In a bearing device of the invention, the play prevention member for preventing occurrence of play in the fit portion of the torque transmission means has an outside diameter larger than an inside diameter of a hollow portion provided in the hub ring and it is a diameter-expanding member to be pressed into the hollow portion. Pressing the diameter-expanding member into the hollow portion of the hub ring expands the hub ring from the inside toward the outside, and thereby a fit condition of the torque transmission means between the hub ring and the joint outer ring can be changed to a tight one.
In a bearing device according to the invention, the hub ring and the joint outer ring can be fitted tighter by adding a constitution in which an end portion of the hub ring is roll-formed to be coupled with the joint outer ring or by adding a constitution where a snap ring is installed at an end portion of the hub ring to couple the hub ring with the joint outer ring.
The present invention is applicable to a constitution in which the hub ring has a small-diameter end portion, an inner ring as a separate element that forms one of double-row inner raceways of a bearing is pressed onto the small-diameter end portion, the hub ring and the bearing are unitized together by roll-forming the small-diameter end portion, and a joint outer ring is fitted into a bore of the hub ring through the torque transmission means.
In a bearing device according to the present invention, the play prevention member for preventing a clearance occurring in the fit portion of the torque transmission means has an outside diameter larger than an inside diameter of the hollow portion of the joint outer ring and is a diameter-expanding member to be pressed into the hollow portion. The diameter-expanding member expands the joint outer ring from the inside toward the outside by pressing the diameter-expanding member into the hollow portion, the fit condition in the torque transmission means between the hub ring and the joint outer, can be changed to a tighter one.
The diameter-expanding member is preferably a rolling element for a bearing. Using a rolling element as a diameter-expanding member facilitates the expansion of the hub ring or the joint outer ring from the inside toward the outside by pressing the diameter-expanding member into the hollow portion. A ball or a roller may be used as the rolling element.
In a bearing device according to the present invention, a small-diameter portion having an outside diameter smaller than an inside diameter of the hub ring is provided at an end portion of the joint outer ring, a portion having projections and depressions is formed on an outside periphery of the small-diameter portion in a circumferential direction, and between an inner periphery of the hub ring and an outer periphery of the small-diameter portion, there is installed a ring member integrally having tongue pieces elastically contacting with the portion having projections and depressions on the small-diameter portion and with the fit portion of the torque transmission means of the hub ring. Installation of the ring member between the hub ring and the small-diameter portion provides a fit condition in which the tongue pieces elastically contact with both the fit portion of the torque transmission means of the hub ring and the portion having projections and depressions of the small-diameter portion, of which arrangement creates a tight fit condition of the hub ring and the joint outer ring even play exists in the fit portion of the torque transmission means.
It is another object of the invention to make a bearing device more compact and to improve its rigidity against moment load.
In order to attain the object, a bearing device according to the invention comprises a hub ring, a constant velocity universal joint and a double-row bearing, of which all are unitized, and one of double-row inner raceways of the bearing is formed on a joint outer ring of the constant velocity universal joint, wherein the joint outer ring is fitted onto the hub ring through torque transmission means, and the hub ring and the joint outer ring are joined by plastic deformation.
By fitting the joint outer ring onto the hub ring as described above, a dimension of the torque transmission means in a radial direction can be made larger than an inside diameter of the hub ring. As a result of the enlargement in the diameter of the torque transmission means, the length of the torque transmission means in an axial direction can be shortened to make the bearing device more compact.
In this case, because the hub ring becomes possible to be extended to inboard side in a bore of the joint outer ring, the double-row bearing, or both inboard and outboard bearings, can take moment load acting on the hub ring through the extended portion, so that moment rigidity of the bearing device is increased. Taking moment load by the double-row bearing reduces load on the plastically deformed portion, so that loosening of the plastically deformed portion is surely prevented, and coupling strength between the hub ring and the joint outer ring is stably maintained over a long period of time. In order to obtain the effect described above, an inboard end portion of the hub ring is preferably extended beyond an inboard inner raceway of the double-row inner raceways.
The torque transmission means and the plastically deformed portion may be separately disposed in an axial direction. In this case, a torque transmission function and a function of preventing the hub ring from loosening are available independently from each other, allowing optimum design of both the functions such that each of the functions can be individually enhanced.
By providing a fit portion between the hub ring and the joint outer ring in which they are fitted without play, transmission of radial load between the two are ensured and radial rigidity of the bearing device is improved. Different embodiments may be available for the arrangements of the fit portion, the torque transmission means and the plastically deformed portion. For example, the plastically deformed portion can be disposed in an axial direction at one side of the torque transmission means and the fit portion can be disposed at the other side.
The plastically deformed portion is arranged such that, for example, an end portion of the hub ring is roll-formed for coupling with the joint outer ring. More specifically, for example, a tubular portion to be roll-formed is formed at the end portion of the hub ring, the tubular portion to be roll-formed is coupled with an end surface of the joint outer ring by plastically deforming the tubular portion to be roll-formed toward the outside of the hub ring by roll-forming or other means, and thus the hub ring and the joint outer ring are coupled together by plastic deformation.
Coupling constitutions for the hub ring and the joint outer ring include, besides those described above, such a constitution in which the hub ring and the joint outer ring are coupled by plastically deforming an end portion of a connection member inserted into the hub ring. The plastically deformed portion and the torque transmission means may be separately disposed in an axial direction, and they may also be disposed on a common part.
More specifically, for example, a portion with projections and depressions is provided within a fit range of the hub ring and the joint outer ring, and the fit range including the portion with projections and depression is at least partly expanded or shrunk in the diameters. By expanding or shrinking the diameters in the fit range, the portion with projections and depression provided either on the hub ring or on the joint outer ring bites into the mating member, a function of torque transmission and a function of preventing the hub ring (or the joint outer ring) from loosening are compatible.
The diameter-expanded portion or the diameter-shrunk portion in this case is preferably disposed on an inboard inner raceway rather than on an outboard inner raceway. By doing so, the dimension of the joint outer ring in an axial direction can be shortened, decrease in accuracy of the flange end surface of the hub ring caused by punching is avoided, and significant rise in costs for a heat treatment for the portion with projections and depression is avoided.
Fitting an inner ring having an outboard inner raceway on its outside periphery onto the hub ring can omit an outboard inner raceway from the hub ring, and the hub ring can be made as a member having no inner raceway. Accordingly, in selecting a material for the hub ring, consideration for hardenability or the hardness of an inner raceway becomes unnecessary. Therefore, material such as extremely purified steel or high-carbon steel is not necessary to be used for the hub ring, so that production costs can be lowered. The same effect is obtainable by fitting an inner ring having an outboard inner raceway on its outside periphery onto the joint outer ring.
For example, the diameter of an outboard outer raceway becomes smaller {circle around (1)} by making the pitch-circle diameter of an outboard bearing of a double-row bearing smaller than that of an inboard bearing or {circle around (2)} by making the diameter of outboard rolling elements smaller than that of inboard rolling elements, and thus the diameter of the outboard outer ring can be made smaller. Therefore, lightening in weight is achieved, space for removing hub bolts is secured, and freedom in design is increased.
The constitutions of {circle around (1)} and {circle around (2)} described above are applicable not only to a bearing device for a driving wheel, but also to a bearing device for a driven wheel. That is, in a bearing device in which a hub ring and a double-row bearing are unitized, the pitch-circle diameter of an outboard bearing of the double-row bearing is made smaller than that of an inboard bearing or the diameter of outboard rolling elements of the double-row bearing is made smaller than that of inboard rolling elements.
It is still another object of the present invention to prevent occurrence of run-out of a wheel mounting flange of a hub ring.
In order to attain the object, a bearing device according to the invention comprises a hub ring, a constant velocity universal joint and a double-row bearing which are unitized together, at least one of inner raceways of double-row inner raceways of the bearing and a wheel mounting flange are formed on the hub ring, and a stem portion of a joint outer ring of the constant velocity universal joint is inserted into the hub ring through torque transmission means, wherein a pilot member, which is a separated element having a wheel guide portion and a brake guide portion, is fitted to an end portion of the wheel mounting flange of the hub ring, and the hub ring and the pilot member are fixed by roll-forming to the joint outer ring. The hub ring, the pilot member and the joint outer ring are fixed to each other by unitizing the hub ring and the pilot member by roll-forming an axial end portion of the joint outer ring to the pilot member.
The constitution described above is applicable to a bearing device having a constitution in which the other inner raceway of the double-row bearing is formed directly on the joint outer ring of the constant velocity universal joint. Also, the stem portion and a mouth portion of the joint outer ring can be constituted so as that torque can be transmitted and also can be detachably constituted. In this case, preferably, the other inner raceway of the double-row inner raceways of the bearing is formed directly on the stem portion of the joint outer ring, and the stem portion and the mouth portion, which is a separate element, are fitted to each other through serrated portions.
In the bearing device according to the present invention, by fitting the pilot member, which is a separate element, to an end portion of the wheel mounting flange of the hub ring, alignment of the hub ring and the joint outer ring is made by the end portion of the wheel mounting flange and the separate pilot member, and the pilot member and the hub ring are fixed to be unitized by roll-forming an axial end of the joint outer ring. The alignment of the hub ring and the joint outer ring is made through the separate pilot member and the end portion of the wheel mounting flange, so that occurrence of run-out of the wheel mounting flange of the hub ring can be prevented. Further, fitting the pilot member eases stress concentration, caused by moment load applied to the hub ring, on a roll-formed portion at the axial end.
In this bearing device, it is preferable that {circle around (1)} the pilot member is subjected to rust-proofing, {circle around (2)} the surface thereof is hardened by a heat treatment, and {circle around (3)} a seal member is provided on a surface thereof with which the hub ring contacts.
It is still a further object of the present invention to enhance the strength of a hub ring while aiming at the lightening in weight without changing the shape and dimensions of a wheel mounting flange and without increasing a carbon content in a material or without adding strengthening elements to a material.
In order to attain the object, a bearing device according to the invention comprises a hub ring, a constant velocity universal joint and a double-row bearing that are unitized together, at least one of inner raceways of double-row inner raceways of the bearing and a wheel mounting flange are formed on the hub ring, and a stem portion of a joint outer ring of the constant velocity universal joint is fitted into the hub ring through torque transmission means, wherein a surface-hardened layer is formed at least on an outboard base portion of the wheel mounting flange. The base portion is preferably a fillet located at a region extending from a brake rotor mounting surface to a tubular pilot portion, and the surface-hardened layer is formed by induction hardening.
Forming the surface-hardened layer at least on an outboard base portion of the wheel mounting flange facilitates strengthening of the outboard base portion, which is the weakest portion for rotational bending fatigue strength, to improve the strength for rotational bending fatigue strength without changing the shapes and dimensions of the wheel mounting flange and without increasing a carbon content in a material or adding strengthening elements in a material, but while enabling the use of current processing methods and facilities. As a result, ribbing or thinning of the wheel mounting flange can be realized, leading in miniaturization and weight-lightening of a bearing device.
In the invention, the surface hardness of the surface-hardened layer may be from HRC 40 through 63, preferably from HRC 58 through 63, and the depth may be from 0.3 through 2 mm, preferably from 0.5 through 2 mm.
When the surface hardness of the surface-hardened layer is made in the range from HRC 40 through 63, or preferably from HRC 58 through 63, the hardness in of range effectively prevents breakage caused by rotational bending fatigue, because rotational bending fatigue strength fundamentally depends on hardness. To prevent breakage caused by rotational bending fatigue means to limit the production of cracks on the material surface. Stress caused by rotational bending has its maximum value at the surface and it reduces toward inside. Therefore, the depth of the surface-hardened layer is not necessarily so deep, and the depth from 0.3 through 2 mm, preferably from 0.5 through 2 mm, is adequate.
The hub ring is preferably made of carbon steel containing C from 0.5 through 0.8 wt %. Carbon steel containing C from 0.5 through 0.8 wt % has better workability than SUJ2, high-carbon chrome steel for a bearing (containing C from 0.95 through 1.10 wt %), by a degree corresponding to its less carbon content.
The hub ring comprises the wheel mounting flange on its outside periphery, an outboard inner raceway of the double-row inner raceways thereon, and a small-diameter stepped portion onto which an inner ring is pressed, with the inner ring being formed with the inboard inner raceway. The present invention is applicable to a bearing device comprising a hub ring having such a constitution described above.
A wheel bearing device preferably comprises a seal having a seal lip slidingly contacts with an outside periphery of the hub ring or the inner ring, and a surface-hardened layer is formed about a base portion of the wheel mounting flange with which the seal lip slidingly contacts. By forming a surface-hardened layer also about a base portion of the wheel mounting flange with which the seal lip contacts, the wheel mounting flange is further strengthened.
It is preferable that a serrated portion is formed on an internal periphery of the hub ring and a surface-hardened layer is provided on the serrated portion. Forming a surface-hardened layer on a serrated portion improves a wear resistance and strength, and the improved strength allows to shorten the effective length of the serrated portion. Here, a serrated portion implies serration or spline.
The nature, principle, and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters.